Referring to FIG. 1, there is shown a common basic circuit of the high voltage generating apparatus used in television receiving units and general display units respectively using a cathode-ray tube.
This basic circuit has the horizontal output circuit 1 and the high voltage circuit 2. The horizontal output circuit 1 comprises the horizontal output transistor 3, damper diode 4, resonance capacitor 5, horizontal deflection coil 6 and the S-shaped compensation capacitor 7. The horizontal output transistor 3 performs switching operation with a voltage pulse transmitted from the horizontal drive circuit not shown. In this case, the current which flows through the transistor 3 becomes a sawtoothed waveform current in the horizontal deflection coil 6 in response to the function of the damper diode 4.
On the other hand, the resonance circuit comprising the resonance capacitor 5 and the horizontal deflection coil 6 generates a flyback pulse by its resonating action and supplies it to the high voltage circuit 2.
The high voltage circuit 2 comprises the flyback transformer 8 and the high voltage rectifying diode 10. One end terminal of the low voltage coil (primary coil) 12 of the flyback transformer 8 is connected to the collector of transistor 3 and the other end terminal of the low voltage coil 12 is connected to the input power supply 13. Meanwhile, the high voltage side terminal of the high voltage coil (secondary coil) of the flyback transformer 8 is connected to the anode 16 of cathode-ray tube 15 through the high voltage rectifier diode 10 and the low voltage side terminal is connected to the ABL (Automatic Brightness Limiter) circuit. In such a configuration, the high voltage circuit 2 boosts a flyback pulse supplied from the horizontal output circuit 1 through the flyback transformer 8, rectifies it by the high voltage rectifying diode 10 and supplies the rectified output E.sub.H to the anode 16.
In case of the high voltage generating apparatus configured as described above, however, the effective output is as large as several tens of watts with a possibility of an accident or a problem resulting from excessive heat generation, burning, etc. if the apparatus is not appropriately designed and manufactured.
The causes of such an accident or problem in design can be enumerated, such as abnormal heat generation of the core 11 and the high voltage coil 14 of the flyback transformer 8 and withstanding voltage failure between coils 12 and 14 and between the layers of the coil. Also problems arise in the manufacturing stage such as shorting of the winding due to miswinding of coils 12 and 14 and internal discharging due to a mistake in insulation of coils 12 and 14.
Although the design and manufacture of the high voltage generating apparatus are controlled so that no error will occur, complete elimination of errors is difficult even with the best care. Therefore, for example, one measure of prevention which can be considered is that a thermal fuse, connected in series to the low voltage coil 12 or the input power supply 13, is provided in the flyback transformer 8 to avoid a fire or accident by melting the fuse with an abnormally high heat when the flyback transformer generates an abnormally high heat and by preventing application of the voltage to the flyback transformer 8.
However, such a problem takes place as discharging occurs between the thermal fuse, if provided, and the high voltage coil 14 when the thermal fuse is arranged near the coil 14. Insulation to avoid such discharge is extremely difficult since there is an extremely large potential difference between the thermal fuse and the coil 14. To prevent a discharge, it is also considered that the thermal fuse should be arranged away from the high voltage coil 14. If it is so arranged, a disadvantage takes place since abnormal heat due to shorting of the winding of the high voltage coil 14 is not smoothly conducted to the thermal fuse, a safety action (fusing of the thermal fuse) is delayed and therefore a fire or the like cannot be prevented. The driving frequency of the flyback transformer 8 generally ranges from 15.75 kHz to 130 kHz and the electromagnetic coupling effect of the low voltage coil 12 and the high voltage coil 14 should be strengthened. If a large profiled member such as the thermal fuse is provided inside the flyback transformer (for example, between both coils 12 and 14), the electromagnetic coupling effect deteriorates to worsen the basic performance of the flyback transformer 8.
The present invention is intended to solve these problems of the conventional apparatus and to provide the high voltage generating apparatus which provides a device capable of quickly and certainly detecting an abnormality due to shorting of the winding of the flyback transformer and preventing a problem or accident such as fire or burning and rarely adversely affects the basic performance of the flyback transformer with provision of this device.